There’s no need to sign up to a gym. Maintain fitness with lots of brisk walking, says the top doctor.

Cutting sugar and refined carbohydrates, like cakes, pasta and pizzas, is also key to good health.

His findings are detailed in his book The Pioppi Diet, which promises to increase longevity by 10 years.

Dr Malhotra said: “It is clear that diet and lifestyle are far more powerful than drugs or taking to the gym.

“Lifestyle changes such as consuming less sugar are more powerful than any medication doctors can prescribe. You cannot outrun a bad diet – this should be what doctors are advising their patients. Eating junk food and then trying to beat the bulge with heavy workouts is a waste of time.”

Enjoying dark chocolate and a glass of wine in moderation, however, would be beneficial to health, said Dr Malhotra, who will be giving a keynote speech to members of the European Parliament in February as part of his campaign to halt the growing obesity crisis.

Dr Malhotra, a founding member of Action on Sugar, a pressure group run by senior doctors, will also reveal his health and diet plan to the Exercise and Medicine Symposium in Cardiff next month.

He says following his diet and lifestyle techniques can transform a person’s health in just 21 days, reducing the risk of heart disease and diabetes and increasing longevity.

His work is largely based on the study of the inhabitants of Pioppi, a fishing village in Italy, where the average age is almost 90 and only a small minority suffer conditions such as heart disease and type 2 diabetes.

Dr Malhotra’s advice comes ahead of a major government health campaign to be launched next week which will encourage parents to reduce children’s intake of sugary snacks to two a day and each of which is no more than 100 calories.

The Change4Life campaign, being launched to help reduce childhood obesity, follows research showing children eat at least three times more sugar than is recommended.

Jack Winkler, Emeritus Professor of Nutrition Policy at London Metropolitan University said: “This is both radical and sudden and looks more aspirational than feasible.”